1. Technical Field
This invention generally relates to test equipment and more specifically relates to equipment for testing properties of materials.
2. Background Art
Electronics have become essential to our modern way of life in the United States. Electronic assemblies are typically made by installing individual components into a printed wiring board (PWB), which are then soldered into place. The PWB makes all the connections between electronic and electrical components using metal paths that are typically etched into the PWB. Modern PWBs have become very sophisticated and complex, and it is not uncommon to have a PWB today that has in excess of twenty layers. Each layer defines conductor paths that connect to one or more other layers. The many different layers allow packing the components very tightly onto a PWB, thereby reducing the overall area of the PWB. This minimization in size of an electronic assembly is essential for many applications where the size of the electronic assembly must be kept very small, as in mobile phones and other hand-held electronic devices.
The reliability of an electronic assembly is directly related to the reliability of the PWB. The reliability of a PWB depends on the stresses that the PWB has undergone. While a PWB undergoes stresses while it is being manufactured, the stresses of particular interest are the stresses to the PWB after it is manufactured that occur during processing of the electronic assembly that includes the PWB. After a multilayer PWB is made, it undergoes numerous processing steps. For example, the components are first incorporated into the PWB. High capacity operations use robots to place the components in their proper location on the PWB. The repeated pressure of placing the components on the PWB stresses the PWB. The soldering of the components also stresses the PWB. An electronic assembly typically undergoes burn-in testing at relatively-high temperatures to catch any parts that suffer from early failures. Burn-in testing causes more stress in the PWB. In addition, many electronic assemblies are tested across a temperature span for several cycles, which causes more stress in the PWB. For some electronic assemblies, the stresses induced into the PWB by the manufacturing process can significantly shorten the life of the electronic assembly. For this reason, testing for residual stress in PWBs has become the focus of increased attention in recent years.
Various methods have been proposed to measure residual stress in a printed wiring board. Non-destructive tests have been developed, which include X-ray analysis and neutron diffraction. However, these methods are not in widespread use for measuring residual stress in PWBs because they are not sufficiently accurate and are not well adapted to a high volume manufacturing environment. Other destructive tests have been developed that more accurately indicate the residual stress in a PWB, but these tests result in the destruction of the PWB. One example of destructive testing drills a hole in the PWB after placing strain gages on the PWB in proximity to the hole that allow measuring the strain before the hole is drilled and after the hole is drilled, and deriving from the changes in strain the residual stress of the board. The theory behind the hold drilling approach is that drilling the hole creates room for the PWB to xe2x80x9crelaxxe2x80x9d, thereby-relieving stress in that location. Another destructive test flexes a PWB that has not undergone manufacturing processes until it fails (i.e., cracks or breaks), the repeats the test on a PWB that has undergone manufacturing processes. By comparing the force required to break the PWB both before and after manufacturing processes, an estimate of residual stress can be derived, but this test again results in the destruction of the PWB. With some modern PWBs, such as motherboards in sophisticated computer systems, the expense of, the PWB is significant, and periodically destroying the boards to test for residual stress is not an acceptable solution. Without a way to accurately measure residual stress in a PWB without destroying the PWB, either the expense of testing for residual stress will continue to be excessive, or PWBs will not be adequately tested for residual stress.
According to the preferred embodiments, an apparatus measures residual stress in a sample under test by measuring the penetration of an indenter on a unprocessed sample under test, and after processing of the sample under test, measuring again the penetration of the indenter on the processed sample under test, and deriving from the two penetration measurements the residual stress in a sample under test. The apparatus and method of the preferred embodiments are especially useful in determining residual stress in a printed wiring board. In this manner a direct measurement of residual stress is possible without destroying the printed wiring board.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.